Bark beetles are insects in the family Scolytidae (Order Coleoptera) that mine in the bark of the bole of many species of trees; certain species are capable of killing their hosts, by mechanically girdling the tree, by inoculating a lethal fungus in to the tree, or both. Bark beetles in this instance are considered to be separate from beetles in the same family that mine in other parts of coniferous trees, e.g. cones, twigs and wood.
Repellents in this instance are defined as agents that impede or prevent successful attack by bark beetles on host logs, stumps, trees or stands through interference with the natural responses by these insects to attractive host volatiles and beetle-produced pheromones. Synonyms include: disruptants, deterrents, interruptants, and inhibitors.
Chemical signals are very important in regulating the behavior of conifer-infesting bark beetles. Attraction to and mass-attack of uninfested host trees is mediated by blends of volatile compounds from the bark of trees in combination with aggregation pheromones produced by either or both sexes of attracting beetles (Borden 1985; Byers 1989). For example, attack by the mountain pine beetle, Dendroctonus ponderosae, is mediated by a blend of host tree monoterpenes, principally myrcene, in combination with the aggregation pheromones, trans-verbenol and exo-brevicomin, produced by attacking females and males, respectively (Borden et al. 1987). Both sexes respond to this attractive composition. As the bark becomes fully occupied, the attacking beetles produce the antiaggregation pheromone, verbenone, which disrupts response to the above attractants (Ryker and Yandell 1983). In so doing, it deters further attack on a tree (Borden and Lindgren 1988), and causes incoming beetles to shift their attack toward neighbouring trees.
Numerous attempts have been made to find inexpensive volatile compounds or compositions that can effectively disrupt the host selection and mass-attack behavior of conifer-infesting bark beetles (Borden 1996). These substances could then be used to protect individual trees or stands from attack. The substances tested include antiaggregation pheromones, pheromones of competing species, green leaf volatiles and host tree constituents.
Two antiaggregation pheromones have shown the potential for practical disruptant activity against conifer-infesting bark beetles. 3,2-MCH (3-methylcyclohex-2-en-1-one) produced by the Douglas-fir beetle, Dendroctonus pseudotsugae, caused a significant reduction of the response of both sexes to attractant-baited traps (Rudinsky 1973). When dispensed aerially in a granular formulation, 3,2-MCH effectively protected vulnerable host logs from attack (Furniss et al. 1981, 1982; McGregor et al. 1984). 3,2-MCH is also produced by the spruce beetle, Dendroctonus rufipennis. In field experiments, 3,2-MCH strongly repelled spruce beetles from attractant-baited traps and from logs treated with vials, granules or bubble caps releasing 3,2-MCH (Kline et al. 1974; Rudinsky et al. 1974; Lindgren et al. 1989a). The antiaggregation pheromone, verbenone, effectively disrupted attack by the mountain pine beetle when deployed in stands of lodgepole pine in slow-release devices affixed to trees (Amman et al. 1989, 1991; Lindgren et al. 1989b, 1994), or broadcast in a granular formulation from the air (Shea et al. 1992). However, the efficacy of verbenone against the mountain pine beetle was inconsistent between years, geographic locations, and tree species (Bentz et al. 1989; Lister et al. 1990; Gibson et al. 1991; Shea et al. 1992). The inconsistency of verbenone is accentuated by its conversion to an insert substance, chrysanthanone, on exposure to ultraviolet light (Kostyk et al. 1993). Against the southern pine beetle, Dendroctonus frontalis, verbenone was effective in controlling spot infestations when trees at the advancing front were treated with verbenone, and freshly-attacked trees were felled, causing both olfactory and visual disruption (Payne and Billings 1989; Payne et al. 1992).
A third pheromone, conophthorin, (E)-7-methyl-1,6-dioxaspiro[4.5]decane, acts as a repellent for male cone beetles, Conophthorus species (Birgersson et al. 1995; Pierce et al. 1995), and as an antiaggregation pheromone for the ash bark beetle, Leperisinus varius (Kohnle 1985). It has not been found in any species of conifer-infesting bark beetle, nor has it been tested for bioactivity against any such beetle.
In some cases, pheromones of bark beetles that compete with the target species for the inner bark can act alone or enhance the repellency of an antiaggregation pheromone. For example, when ipsdienol, a pheromone common to Ips species was deployed with the antiaggregation pheromone, verbenone, the two compounds acted synergistically to disrupt response by the western pine beetle, Dendroctonus brevicomis, to attractant-baited traps and ponderosa pine trees (Paine and Hanlon 1991; Bertram and Paine 1994a,b). Similarly, verbenone produced by the mountain pine beetle and ipsenol produced by Ips latidens acted synergistically to disrupt attack on felled lodgepole pine trees by the pine engraver, Ips pini (Borden et al. 1992; Devlin and Borden 1994).
Another source of repellents lies in green leaf volatiles, six-carbon alcohols, aldehydes and derivative esters commonly found in green plants (Visser 1986). Two green leaf volatiles, 1-hexanol and hexanal, were shown to be moderately effective disruptants of the pheromone response of the southern pine beetle and two Ips species (Dickens et al. 1992, 1993, 1995). When a wider array of green leaf volatiles was tested against the mountain pine beetle, hexanal was found to be inactive, 1-hexanol was a moderately effective repellent, and two other compounds, (E)-2-hexen-1-ol and (Z)-3-hexen-1-ol, were very effective in repelling both sexes from attractant-baited traps and trees (Wilson et al. 1996).
Of the host tree compositions tested as attack disruptants for conifer-infesting bark beetles, one of them, pine oil, is a crude mixture of monoterpenes and many other unknown constituents. When sprayed to the drip point onto the lower bole of attractant baited trees pine oil disrupted attack for varying periods of time by the mountain pine beetle (Nijholt et al. 1981; McMullen and Safranyik 1985; Richmond 1985), the spruce beetle (Nijholt et al. 1981; Werner et al. 1986), the Douglas-fir beetle (Nijholt et al. 1981), and the southern pine beetle (O'Donnell et al. 1986). However, spraying the bole with pine oil was not effective in preventing attack by the black turpentine beetle, Dendroctonus terebrans, or by the eastern fivespined ips, Ips grandicollis (Berisford et al. 1986), nor was it recommended as an area-wide operational treatment for the mountain pine beetle (Borden et al. 1988).
The other host tree volatile with practical potential as a disruptant is 4-allylanisole (also known as estragole and methyl chavecol), a compound that comprises about 1.0% of the xylem oleoresin of loblolly pines (Strom et al. 1996). At generally very high doses, e.g. 160 mg per 24 h, 4-allylanisole and three of its analogs (Hayes et al. 1995 a, b) disrupted the responses to attractant-baited traps by the southern pine beetle (Hayes et al. 1994), the mountain pine beetle (Hayes and Strom 1994; Hobson 1995), the western pine beetle (Hobson 1995), and the pine engraver (Hayes and Strom 1994). A much lower dose of 0.1 mg per 24 h was effective for the spruce beetle and the eastern larch beetle, Dendroctonus simplex (Werner 1995). When deployed from open vials suspended from ropes hung vertically on the boles of southern pines, 4-allylanisole provided effective protection against the southern pine beetle (Strom et al. 1995; Hayes et al. 1996).
Schroeder (1992) showed that if bolts of aspen, Populus tremula, or birch, Betula pendula, were hung beside traps baited with ethanol, the European bark beetles, Tomicus piniperda and Hylurgops palliatus, were repelled. However, in no case has there been a scientific investigation of the means by which conifer-infesting bark beetles use specific volatile chemicals to detect and avoid non-host angiosperm tree species. In nature, attack of these trees would result in death of the attacking beetles. We reasoned that the beetles' life or death decision whether or not to attack a given tree is so important, that we would be likely to discover new repellents for bark beetles in the volatiles emitted by non-host angiosperm trees.